Additionally, if you open any grouped obj in a text editor grouped polys will be identified by a g header tag, materials by a usemtl... this could be useful for grouping and material manipulations. For instance I could assign my potential groups as materials and vice versa via editing these definitions in the obj file... so in a modeler I could select the polys that I wanted to be the a named group and assign a material like named to those polys and then edit the obj file to reflect this with a g tag, or any combination of manipulations like this...